RESPONSIVENESS IN VERTEBRATES 



Muscles are attached to rigid bones which are held together at the joints and 

 are characterized by their abiUty to contract or shorten. By puUing against 

 one another across joints, muscles make possible typical postures, as in man, 

 and when they contract produce locomotion or move the appendages inde- 

 pendently. Joints of the skeleton of the appendages are freely movable and 

 are of two main types. Hinge joints, which are found between the bones of 

 the upper and lower arm and upper and lower leg, allow movement in only one 

 plane. Ball-and-socket joints, which are found where the bone of the upper 

 arm articulates with the shoulder and the bone of the upper leg articulates 

 with the hip girdle, permit a rotation of the limbs when certain muscles con- 

 tract or pull against others. The coordination of the activities of the muscles 

 is extremely important for the animal as a whole and is brought about by the 

 nervous system. 



The Endocrine System. Reference has been made to glands, or organs 

 of secretion, which possess ducts and pass their secretions onto body surfaces 

 (p. 61). There are a number of glands which do not have ducts by which 

 to discharge their secretions; instead, the secretions pass directly into the 

 blood stream. Such glands are known as the ductless glands, glands of internal 

 secretion, or endocrine glands. Their secretions are called internal secretions, 

 endocrines, or hormones. The endocrine system does not consist of a group of 

 closely associated organs like the other systems. Instead, the endocrine glands 

 are widely separated from one another and possess only one feature in com- 

 mon; they pass their secretions into the blood stream. The principal organs 

 of the endocrine system are the hypophysis (pituitary gland), thyroid gland, 

 parathyroid glands (four in man), pancreas, adrenal glands, the gonads, and, 

 in the higher mammals, the placenta (Fig. 4.8). There are, however, several 

 more or less isolated groups of cells, especially in the wall of the small 

 intestine, which also produce hormones. 



Coordination with Special Reference 

 to the External Environment 



Basic Mechanisms of Nervous Coordination. Every nervous coordi- 

 nation is the result of a reaction by some part of the body to a stimulus. The 

 simplest type of response to a stimulus is known as reflex action. When you 

 touch anything hot with your finger, the muscles of your arm react to with- 

 draw your hand. A nerve impulse passes from the point stimulated to the 

 central nervous system and travels back to produce the contraction of the arm 

 muscles. Another well-known illustration is the knee-jerk reflex, in which the 

 leg is extended as a result of a sharp tap below the kneecap. In both these 

 examples the response to the stimulus is apparent in the general region that 

 receives the stimulus. Complete analysis of these simple reflexes shows that 

 they are the expression of a nervous mechanism which some investigators be- 

 lieve explains all nervous coordination. 



95 



